When we say human nature can decide what is moral we are essentially saying everyone can do what is right in their own eyes. There is no right or wrong.
Everyone knows it is wrong to murder but we simply don’t wish to acknowledge it. On something more closer to home, how many of us would tell our kids it’s alright to lie to us or steal from us?
Yes, there really is a code of moral behavior written on our hearts and in scripture. We just want to choose it whenever it’s convenient to us.
It is inherent in the hearts of every human being, but without learning from the outside, and societal standards that also agree, humans fail miserably as individuals, and society devolves to hellishness.
That is what we are witnessing right now.
The rejection of absolute truth means what was stated above: right and wrong are decided by the mob, the elites, or each chooses his own. But each choosing his own never lasts long, the big dogs take over right quick.
That is why those who posit that there is no absolute right and wrong and everyone is free to choose, invariably force those who don’t agree with them into compliance. They really do believe in absolutes, but their own. People with this view set themselves as the supreme arbiter. When they reject God, it is due to envy, and they try to take His place.